Synthesis of beryl



Oct. 21, 1969 D MLSON ETAL 3,473,935

SYNTHESIS OF BERYL Filed Aug. 18, 1965 1 Wayne 0. Wiison x a v t ,3 3Hubert 5. Hall :5 INVENTORSA I. M;

11 BY ATTORNEY AGENT.

United States Patent 3,473,935 SYNTEESIS 0F BERYL Wayne D. Wilson, 2000Wallace Ave., Silver Spring, Md.

20902, and Hubert B. Hall, 716 Somerset Place, Hyattsville, Md. 20783Continuation-impart of application Ser. No. 377,781, June 24, 1964. Thisapplication Aug. 18, 1965, Ser. No. 482,035

Int. (3]. C041) /44 US. Cl. 106-42 21 Claims ABSTRACT OF THE DISCLOSUREA method of making beryl crystals by compressing beryl powder at 7.5 to15 kilobars at the melting temperature of beryl. The melting temperaturemay be lowered by the use of a flux such as water or a fluorine source.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This application is a continuation-impart of application Ser. No.377,781 filed June 24, 1964, for Synthesis of Beryl, now abandoned.

This invention pertains to the formation of crystalline materials andmore particularly to a method for producing beryl crystals.

Beryl crystals have previously been formed by several different methodseach of which has its own peculiar disadvantages. The growth of singlecrystals from a seed crystal suspended in a melt suffers from thedisadvantage that it requires too long a time (probably in the order ofa week or more to produce a sizeable crystal) and the Verneiul flamefusion process, although requiring less time, suffers from thedisadvantage that it is diflicult to maintain the necessary precisecontrol over the feed rate of the constituents employed. Moreover whenthese processes have been employed for the production of emeraldcrystals, the emeralds produced have a density of 2.67 or lower, whichis considerably lower than the density of natural emerald (2.70 andslightly higher than 2.71 for the purest natural emerald). Thesesynthetic emeralds also have a series of wisp-like inclusions which arevisible to the naked eye and such inclusions seriously affect theoptical applications of the crystals.

Accordingly, it is an object of this invention to provide a new methodfor producing beryl crystals.

It is another object to produce clear single beryl crystals in a shortspace of time.

It is a further object to produce single clear emerald crystals.

It is still another object to produce emerald crystals that havedensity, hardness and optical properties that are superior to previoussynthetic emeralds.

These and many other objects will become more fully apparent from thefollowing detailed description of the invention wherein:

FIG. 1 is a partial cross-sectional view of a steppedcore pressureapparatus for providing the high pressure necessary to the presentinvention, the capsule containing the material to be crystallized beingshown in place; and

FIG. 2 is a cross-sectional showing of the capsule itself.

The objects of this invention are accomplished by forming beryl crystalsunder pressure from either beryl powder or the constituents of beryl intheir proper proportions (3BeO:Al O :6 SiO which combine under theprocess conditions to form beryl). It is to be understood that the termberyl powder is meant to include all mem- Patented Get. 21, 1969 bers ofthe beryl family which includes aquamarine (ferrous ion impurity),morganite (lithium ion impurity), emerald (chromium ion impurity),golden beryl (ferric ion impurity) and goshenite (no coloring impurity).Hereinafter, in referring to the starting material, the term beryl willinclude a proper proportioned mixture of the constituents of beryl andall the members of the beryl family.

In particular the method comprises melting beryl and subsequentlyallowing it to solidify with the melting and solidification steps beingcarried out under pressure. It has been found that if clear transparentcrystals of beryl are heated to temperatures above 600 C., there is amolecular dissociation and self-diffusion of some of the constituents ofberyl through the crystal, said diffusion destroying its opticalproperties. Consequently, if an attempt is made to form beryl crystalsby melting beryl, self-diffusion of the constituents of beryl wouldprevent the formation of clear, transparent crystals. It has been foundthat this problem can be obviated by the application of pressure duringthe melting and cooling steps.

The pressure is only critical in that it must be high enough to preventself-diffusion, yet not so high that it prevents the beryl fromexpanding during solidification. Beryl has a negative coefficient ofexpansion in a direction parallel to the hexagonal crystal axis and thusduring solidification it will expand in one direction and contract inthe other. If the pressure is so high that this expansion can not occur,no crystals will be formed. Although beryl crystals can be formed at anypressure that is sufficient to both prevent self-diffusion of the berylconstituents through the crystal and allow expansion of the crystalduring solidification, as a practical matter the minimum and maximumpressures that are used are dictated by the particular pressure systememployed. For example, in the apparatus depicted in FIGS. 1 and 2 whichis described more fully below, it has been found that due to the closedcapsule employed the pressure should not exceed about 20 kilobars andpreferably should not exceed about 15 kilobars. It has been found thatgood results are obtained in this apparatus if the pressure ismaintained between about 7.5 and 15 kilobars.

The temperature employed is any temperature which is sufficient to meltthe beryl without decomposing it (when the constituents of beryl areemployed they melt and combine to form beryl) and of course this meltingtemperature is a function of pressure, with the change being about 40 C.per kilobar (beryl melts at 1410 C. at atm. pressure and 1800 C. at 10kilobars). Once the beryl is uniformly melted, temperature reduction maybe started almost instantaneously and this reduction in temperature maybe effected abruptly or over a period of time without any significantchange in results.

It is also possible to solidfy the beryl after melting by raising thepressure rather than diminishing the temperature. For example, the berylcan be melted at 1800 C. at a pressure of 10 kilobars and then besolidified by raising the pressure, eg to about 20 kilobars whilemaintaining the same temperature or any temperature that is below themelting point of beryl at 20 kilobars.

The high temperatures used in the method of this invention introduceslimitations as to the material employed as the capsule for the beryl. Ifthe capsule material melts at the temperatures employed, it will diffusethrough the beryl preventing the formation of a single clear crystal andtherefore the capsule must be made from a material that does not reactwith or diffuse into the beryl, i.e. it must be inert at hightemperatures. It has been found that carbon in the form of graphite isan excellent capsule material at all pressures (melted beryl has beenheld in such a capsule for as long as one-half hour at 15 kilobarswithout reaction, mixing or dilfusion processes occurring) with carbonlined materials such as carbon lined Vycor glass giving equally goodresults. Capsules formed from:

platinum and tungsten are not as effective since the melting andsolidification must be performed rapidly in order to prevent diffusion.Another means of avoiding the reaction and diffusion problem is toemploy one of the constituents of beryl itself, e.g. BeO as the capsulematerial.

The process of this invention may be carried out in apparatus such as isshown generally as in FIG. 1. This apparatus is of the so-calledstepped-core type and is composed of a plurality of concentric bindingrings 11-15 inclusive, surrounding a tungsten carbide core 16. Core 16has a cylindrical bore 17 therethrough, and counterbores 18 and 19opening into each end thereof. In coaxial alignment with bore 17 are twopress pistons 23 and 24 having tungsten carbide end elements 26 and 27configured to cooperate with counter-bores 18 and 19 and bore 17 inorder to impose pressure on the sample contained within bore 17. Pistons23 and 24 are biased toward one another by apparatus (not shown) whichmay be a hydraulic press as is well known in the art. The particularapparatus used by the inventors is a 300 ton press.

The same container 28 is shown in detail in FIG. 2. The material to :becrystallized in the process is diagrammatically shown at 29 as containedwithin a closed capsule 31. Capsule 31 is disposed within a pressed talccylinder 32 which is in turn supported within a carbon cylinder 33 whichserves as a heat source when electric current is passed therethrough.Disposed within the tale cylinder 32 at each end of the sample capsule31 are pyrophyllite plugs 34r and 35 and surrounding the outside of thecarbon cylinder 33 is a body of pyrophyllite 36 which is configured tofit within bore 17. Completing the container assembly are a pair ofelectrically conductive end caps 37 and 38 which are disposed incounterbores in pyrophyllite cylinder 36 and are in electrical contactwith the carbon cylinder 33. Electrical contact is made to the outsideof end caps 37 and 38 through the press pistons themselves, currentbeing supplied to the press pistons by conductors 39 and 40.Counterbores 18 and 19 are also provided with annular pyrophyllitemembers 43 and 44, which along with the pyrophyllite in the samplecontainer and the pressed talc cylinder serves to transmit pressure tothe sample. With this apparatus it is possible to obtain pressures up to60,000 atmospheres in the capsule.

The process is carried out in the apparatus by first adjusting thepressure to the proper value. The temperature is then raised until thepowder melts by passing an electric current through carbon cylinder 33.The electric current is then abruptly cut off. Since there is a largemass of steel surrounding the sample the temperature is reduced tonearly room temperature in a matter of a few minutes. Under theseconditions, the melt fuses into a single crystal of beryl which isremoved from the press after the pressure has been reduced.

It is to be understood, however, that the method of this invention isnot to be limited to the apparatus described above since the process maybe performed by any means which can effect melting and solidificationunder pressure. It should also be apparent that many modifications canbe made in the apparatus described above without departing from itsgeneral concept, e.g. some or all of the pyrophyllite members may bereplaced by another pressure transmitting material such as boronnitride.

The process of this invention may be employed to form any of thecrystalline members of the beryl family (aquamarine, goshenite, emerald,golden beryl and morganite) by adding the ionic impurity that gives theparticular member its characteristic color. For example, if it isdesired to produce emerald crystals, chromium ions are added to theberyl sample before melting and cooling under pressure, the chromiumions giving the finished crystal the characteristic green color ofemeralds. The chromium ions may be introduced by adding an amount of CrO to the beryl that does not exceed about 2 weight percent, preferablybetween about 0.5 and 1 weight percent with 1 weight percent givingespecially good results.

-- Although the -proeessdescribed above is-Wholly satisfactory for thecreation of large clear single crystals. many modifications can be-madewithout departing from the spirit of the invention. For example, watercan be added to the beryl sample to reduce the temperature coeflicient,said reduction effecting 'a lowering of the process temperatures (1weight percent of water reduces the'melting point of beryl from about2000 C. to about 1500 C. at 15 kilobars). f i I Although water additionhas a beneficial effect on process conditions, an excess of wateradversely effects crystallization and from theoretical calculations ithas been determined that the total amount of water present should notexceed about 6.5 weight percent. Since beryl may already contain somewater (for example, beryl samples used in the form of goshen'ite usually:contain enough water to effect such a reduction without wateraddition), the water content should not exceed about 2 weight percentpreferably not in excess of 1 weight percent.

It has been found that the temperature, of melting can be reduced evenfurther by using fluorine gas as a flux. One way to obtain the fluorineis to use Teflon as a capsule material. At a temperature near 800 C.,the Teflon? dissociates to give off fluorine which causes the beryl tomelt inthe neighborhood of 1000 C. at 20 kilobars pressure. Another wayto obtain the fluorine is to use CrF in place of Cr O as the coloringagent. When this is done, the fluorine gas is given off at 1000 C. at 15kilobars.

In another alternative embodiment, the pressure is established at 10kilobars and the temperature is raised to 1000 C., a temperature wellbelow the melting temperature of the beryl and container material. Theconditions of temperature and pressure must be held for a much longertime under these conditions in order to obtain sizeable crystals. In oneexample, the conditions were held for a period of 25 minutes, resultingin small microscopic crystals of beryl, while the major portion of thesample had the appearance of a gnanular rock. Much longer times arerequired to achieve larger crystals.

Example I Beryl powder (aquamarine) mixed with 2% Cr O and 1% H O wasplaced in a tungsten capsule and subjected to a pressure of 15 kilobarsand a temperature of 1570 C. for two minutes in the apparatus of FIG. 1.After cooling and reducing the pressure, a single clear crystal wasremoved from the capsule which proved to be emerald.

Example II Similarly to Example I, beryl powder (aquamarine) mixed with2% Cr O and 1% H O was placed in a tungsten capsule and subjected to apressure of 15 kilobars and a temperature of 1470 C. for two minutes.After cooling, emerald crystal having a density of 2,716 was removedfrom the capsule.

Example III Similarly to Example I, beryl powder (aquamarine) mixed with1% Cr O and 1% H O was placed in a carbon capsule and subjected to apressure of 15 kilobars and a temperature of 1425 C. for five minutes.After cooling, it was found that single emerald crystal formedthroughout the capsule.

Example IV Similarly to Example I, beryl powder (goshenite) mixed with0.5% Cr O was placed in a carbon lined Vycor glass capsule and subjectedto a pressure of kilobars and a temperature of 1535 C. After cooling itwas found that single emerald crystal formed throughout the capsule.

ExampleV A mixture was prepared by combining 6.98 g. BeO, 9.485 g. A1 0and 35.53 SiO To this mixture there was added 2 weight percent C1'2O3and 2 weight percent H 0 and the mixture was placed in a tantalumcapsule.

The mixture was subjected to a temperature of 2015 C. and a pressure of15 kilobars for two minutes. The temperature was dropped rapidly and asingle emerald crystal was formed in the capsule.

Example VI The mixture employed in Example V was heated at 2000 C. and15 kilobars for one minute in a tantalum capsule. After rapid cooling itwas found that emerald crystal formed throughout the capsule.

The method of this invention may be easily performed in any pressuresystem which is sufiicient to produce a pressure that will preventdiffusion of the beryl constituents during melting and solidification.The opticum conditions employed will vary from system to system and theymay be easily determined by those skilled in the art from the teachingof this disclosure that the starting material may be melted andsolidified under any pressure that prevents diffusion of theconstituents and allows expansion of the crystal.

The method of this invention is extremely effective for producing singleclear beryl crystals. The starting materials, beryl in any of its formsor the constituents of beryl, are readily available and any member ofthe beryl family can be produced by adding the proper coloring agent.Unlike previous processes for forming beryl crystals, the process ofthis invention is extremely rapid since the crystal can be formed asquickly as the starting material can be melted and solidified underpressure.

The method of this invention is especially valuable for the productionof emerald crystals, since this particular form of beryl, as well asbeing a valuable gem, has particular utility in laser and masertechnology. The emeralds produced by this invention have hardness,density, and optical properties that are far superior to any syntheticemeralds that have been produced in the past.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A method for producing beryl crystals which comprises melting amember selected from the group consisting of beryl powder and a mixtureof the constituents of beryl in their proper proportions, andsolidifying said member to produce beryl crystal, said melting andsolidification being performed at a pressure sutficiently high toprevent self-diffusion of the constituents of beryl through the crystalsyet not so high as to prevent expansion thereof during solidification.

2. The method of claim 1 wherein said member is beryl powder.

3. The method of claim 1 wherein said member is beryl powder in the formof goshenite.

4. The method of claim 1 wherein said member is beryl powder in the formof aquamarine.

5. The method of claim 1 wherein said member is the constituents ofberyl in their proper proportions.

6. The method of claim 1 wherein said member contains an amount of waterthat does not exceed about 2 weight percent.

7. A method for producing emerald crystals which comprises, melting amember selected from the group consisting of beryl powder and a mixtureof the constituents of beryl in their proper proportions, said membercontaining chromium ions, and solidifying said member to form emeraldcrystals, said melting and solidification being performed at a pressuresufiiciently high to prevent self-diffusion of the constituents of berylthrough the crystal yet not so high as to prevent expansion thereofduring solidification.

8. The method of claim 7 wherein said member is beryl powder.

9. The method of claim 7 wherein said member is beryl powder in the formof goshenite.

10. The method of claim 7 wherein said member is a mixture of theconstituents of beryl in their proper proportions.

11. The method of claim 7 wherein said member contains an amount ofwater that does not exceed about 6 weight percent.

12. A method for producing emerald crystals which comprises melting amixture of beryl powder and Cr O said Cr O being present in an amountnot exceeding about 2 weight percent and solidifying said mixture toform emerald crystal, said melting and solidification being performed ata pressure sufficiently high to prevent selfdiflusion of theconstituents of beryl through the crystal yet not so high as to preventexpansion thereof during solidification.

13. The method of claim 12 wherein the beryl powder is in the form ofgoshenite.

14. The method of claim 12 wherein the beryl powder is in the form ofaquamarine.

15. The method of claim 12 wherein the mixture contains an amount ofwater that does not exceed about 2 Weight percent.

16. A method for producing emerald crystals which comprises melting amixture of the constituents of beryl in their proper proportions, saidmixture also containing Cr O in an amount that does not exceed about 2weight percent, and solidifying the mixture to produce emerald crystal,said melting and solidification being effected at a pressuresufficiently high to prevent self-diffusion of the constituents of berylthrough the crystal yet not so high as to prevent expansion thereofduring soldification.

17. The method of claim 16 wherein the mixture contains an amount ofwater that does not exceed about 2 weight percent.

18. A method for producing emerald crystals which comprises melting amixture of Cr O and a member selected from the group consisting of berylpowder and a mixture of the constituents of beryl in their properproportions, said Cr O being present in an amount that does not exceedabout 2 weight percent, and solidifying the mixture to form emeraldcrystal, said melting and solidification being performed at a pressurethat ranges from about 7.5 to 15 kilobars.

19. The method of claim 18 wherein the mixture contains an amount ofwater that does not exceed about 2 weight percent.

20. A method for producing emerald crystals which comprises,

(a) placing a mixture of Cr O and a member selected from the groupconsisting of beryl powder and a mixture of the constituents of beryl intheir proper proportions in a capsule, formed from material inert athigh temperatures said Cr O being present in an amount not exceedingabout 2 weight percent,

(b) placing said capsule in a pressure vessel having an electric heatingelement therein,

(0) raising the pressure on said capsule to a pressure between 7.5 and15 kilobars,

(d) passing an electric current through said heating element to heat themixture to its melting point under the established pressure, and

(e) solidifying the-melt to produce emerald crystal: 21. The method ofclaim 20 wherein the capsule is formed from a member selected from thegroup consisting of carbon and carbon lined materials.

References Cited UNITED STATES PATENTS 3/1969 Gordy 106-42 12/1943Cooper 10642 6/1960 Wenturf 10642 8/1961 Wenturf 106-42 FOREIGN PATENTS10/1959 Germany.

OTHER REFERENCES HELEN MCCARTHY, Prin1' u'y Examiner US. Cl. X.R.

